RESPONSIVE CURVATURES— NECxATIVE GEOTROPISM 509 



view that the fundamental action of gravitational stimulus is 

 to increase the rate of growth ; it shows, on the contrary, that 

 contraction under stimulation is the active factor. 



Growth of grass haulms on a klinostat. — I shall here 

 adduce certain considerations which may further serve to 

 explain the difference between the stationary or feebly grow- 

 ing grass haulms, and other normally growing organs, as 

 regards the effects induced in them by the rotation on the 

 klinostat. It is found that in grass haulms, when subjected 

 to the rotation of the klinostat, growth is recommenced, or 

 increased ; while in other normally growing plants there is 

 no such increase of rectilinear growth. For an explanation 

 of this difference we have to recall the effect on growth of 

 increased internal hydrostatic pressure, with the consequent 

 increase of turgidity, which has already been described 

 (p. 428). It was there shown that, when the plant was grow- 

 ing at a moderate rate, the curve of relation between increase 

 of turgidity and increase of growth was practically a straight 

 line ; that is to say, any increase or diminution of turgidity 

 would then produce a proportionate increase or decrease of 

 growth. But this relation did not hold good when the 

 natural rate of growth was feeble or absent. In the latter 

 cases, increase of pressure induced an effect that was dis- 

 proportionately large. And this was specially the case when 

 the growth of an organ had come to a temporary condition 

 of standstill. In that case, when the pressure was gradually 

 increased, growth was found at a certain point to be abruptly 

 renewed, and to go on increasing with the increase of pres- 

 sure, at a rate disproportionately large. If, now, the pressure 

 be once more brought down to what it was just before the 

 point was reached at which growth was started, we find that 

 growth is not arrested, but persists. Thus the net result 

 of these alternations of pressure is a positive resultant 

 growth. We have, thus, two distinct cases: (i) that in 

 which the normal rate of growth is moderate, and in which 

 alternate increase and diminution of pressure, acting for 

 equal lengths of time, will induce equal increase and decrease 



